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recursionschemes

Ah yes I love arrays with a length of infinity!!!
Writing something as a type of fold over an infinite sequence is nicer than using recursion directly in my opinion. See: https://hackage.haskell.org/package/recursionschemes

Tips on mastering recursion and trees and shit?
Consider recursion schemes! It let's you separate the logic of how your recursion is structured on your data, and the logic of what you're doing on each recursion stage. So e.g. you can write the core logic of a recursive linked list summation as just fun x accum > x + accum, and then you just find the appropriate recursion scheme to pipe the list values into x and handle recursing to build accum (a catamorphism in this case)

So you come across an undocumented library…
But wait, there's more! One of the two challenges was the recursionschemes library. I wrote some examples, but the output I was getting from that example was not what I expected. I dug further, and it turned out to be a bug in the library! In addition to the documentation, I thus also worked on a fix for that bug.
It's a pretty complicated bug, documented in details at https://github.com/recursionschemes/recursionschemes/issues/50

Beautiful ideas in programming: generators and continuations
It’s also trivial and easy in Haskell — you just need an instance of `Foldable` or `Traversable` on your collection, and then you can fold or traverse it in a configurable way. Or for recursive structures, use https://hackage.haskell.org/package/recursionschemes. Or even just pass a traversal function as an argument for maximum flexibility.

fromMaybe is Just a fold
https://hackage.haskell.org/package/recursionschemes is the "normal" library for this type of generalized folding. It even contains Base instances for Maybe and Either.

Annotation via anamorphism?
I've been working on a system which uses recursionschemes to annotate a recursive type. The annotated tree itself is pretty simple; at each level, we pair the annotation with the base functor, or

Seeking a Project Lead for Matchmaker  Haskell Foundation
Yes please! Right now all of my opensource projects (most notably hint and recursionschemes) are about to drop into barelyupdated mode, and while I knew this would happen and have been working towards finding comaintainers, I am now realizing that it wasn't enough. I think such a website would definitely have helped, and I am hoping that once it launches, I'll be able to use it to find some comaintainers to tide over my projects until I become available again.

Annotate AST with location information
A short (?) primer before you dive into recursionschemes:

Anamorphisms aka Unfolds Explained  Functional Works
And here is the branch containing my implementation.
What are some alternatives?
pipes  Compositional pipelines
distributedprocessplatform  DEPRECATED (Cloud Haskell Platform) in favor of distributedprocessextras, distributedprocessasync, distributedprocessclientserver, distributedprocessregistry, distributedprocesssupervisor, distributedprocesstask and distributedprocessexecution
daemons  Daemons in Haskell made fun and easy
unliftio  The MonadUnliftIO typeclass for unlifting monads to IO
fusedeffects  A fast, flexible, fused effect system for Haskell
apecs  a fast, extensible, type driven Haskell ECS framework for games
extensibleeffects  Extensible Effects: An Alternative to Monad Transformers
chrcore  Constraint Handling Rules
pipescore  Compositional pipelines
conduitcombinators  Type classes for mapping, folding, and traversing monomorphic containers
record  Anonymous records
rank2classes  Grammar parser  combinator library for parsing a general contextfree grammar